The bulb thermometer is the commonest. The thermometer contains a liquid, generally alcohol or mercury.
Bulb thermometers rely on the simple principle that a liquid changes its volume relative to its temperature. Liquids take up less space when they are cold and more space when they are warm (this principle works for gases and is the basis of the hot air balloon).
We tend not to notice that things like water, milk and cooking oil all take up more or less space as their temperatures change. In these cases, the change in volume is fairly small. All bulb thermometers use a fairly large bulb and a narrow tube to accentuate the change in volume. You can see this for yourself by making your own bulb thermometer from scratch. Here is what you’ll need – and thanks to howstuffworks
- A small glass jar or bottle with a water-tight lid – The lid should be the screw-on kind and made from metal or plastic. The jar needs to be glass so that its shape does not change when you squeeze it.
- A drill or a hammer and a large nail
- Some silly putty, plumbers putty, or even (chewed)chewing gum
- A drinking straw – quite long, the thinner the better, preferably clear
- Some food colouring (not required but makes it easier to see)
To make your thermometer:
- Drill or punch a hole in the lid of your jar. The hole should be as close to the diameter of the straw as you can get.
- Fill your jar with cold water. You can do this either by filling it with water and leaving it in the refrigerator overnight, or by pouring ice water into your jar (straining the ice out in the process – all you want is water in the jar). Add food colouring if you want and shake it up. Put the jar on the table to keep it steady — you want the jar filled to the brim with cold water, as full as you can get it without overflowing.
- Put the lid on the jar as shown in the figure above. When you screw on the cap, a little water may spill out the sides, and a little water may be visible in the straw. That’s okay, don’t worry
- Place the jar in your kitchen sink, plug the sink and run hot water into the sink until the sink is about half full. Watch the level of the liquid in the straw and watch. As the water in the jar gets warmer, it will expand and rise up the straw. This sort of expansion happens every day, but we don’t really notice it because the amount of expansion is fairly small. Here, because we have routed the expanding water into a narrow straw, it is much more obvious. We can actually see it happening.
What you have created is a simple bulb thermometer. And it works pretty well. If you wanted to you could calibrate it, and it would tell you the temperature fairly accurately. This particular thermometer has a few problems, however:
- Because the working fluid is water, it cannot measure temperatures below 00C(the water would freeze). It also cannot measure temperatures above 1000C (the water would boil).
- Because the “bulb” (the jar) is relatively large, it takes a long time for the thermometer to reach the same temperatures as the object it is measuring – perhaps as long a half an hour.
- It can’t follow rapidly changing temperatures, for the same reason.
- It’s big and awkward. We couldn’t use it t measure the temperature at a point.
- Because the top of the tube is open, the water can evaporate and pick up dust and debris.
Sealing mercury in a small glass thermometer solves these problems. The small size of the bulb means that the bulb reaches the temperature of what it is measuring quite quickly, and the tube in such a thermometer is micro-fine – a ‘capillary tube’. Mercury also avoids the freezing and boiling problems associated with water.
How do you calibrate the thermometer? This means putting numbers on it and making a scale. To do this, we need two fixed points, temperatures which we know won’t change. The ice point is one – the temperature of pure melting ice. Immerse your bulb in pure melting ice and leave it long enough to settle down, Make a mark on the clear drinking straw – this is nought (0) degrees Celsius.
Now arrange to suspend your bulb over a pan of boiling water, just above the surface. The liquid will rise up the tube and when it stops ( to a fair approximation) it is measuring the temperature above pure boiling water at standard atmospheric pressure. This is our second fixed point which we can label 100 degrees Celsius. All we then have to do is make a linear scale on the straw from 0 to 100 and we have a way of measuring temperature.
The scale is linear.
- It’s not very sensitive – how could we make it more sensitive?
- It takes a long time to respond – how could we speed up the response time?
- It has a limited range – below 0 the water freezes, above 100 it boils. What could we do to extend the range?